Dry etching is frequently employed during semiconductor manufacturing for forming structures with small feature sizes. In dry etching, gases are the primary etching medium and the substrate, such as a semiconductor silicon wafer, is etched without wet chemicals (i.e., without etching solutions). During dry etching processes, the material being etched is converted into gas phase materials which are removed from the etching chamber by a vacuum system.
One example of dry etching is plasma etching. Plasma etching utilizes a plasma as the etching medium. Etching plasmas contain many different gaseous species, including ions, free radicals, electrons, photons, neutrals and reaction byproducts.
In one known method of plasma etching referred to parallel plate reaction plasma etching, a process chamber (e.g., the plasma etching chamber) is evacuated and a gas mixture is fed into the chamber and energized to a plasma state using a radio frequency (RF) source. The RF source is capacitively coupled to a substrate to be etched and to an electrode, while another electrode or the inner wall of the chamber is grounded. Positive voltages applied to the one electrode are typically on the order of several hundred volts. The gas mixture forms a glow discharge. Glow discharges are non-equilibrium plasmas wherein the electron temperature is greater than the gas temperature and the ratio of electrons to neutral species is typically in the range of 10−6 to 10−4. Under these conditions wherein a glow discharge has been created and the voltage potential between the electrodes is large enough, the substrate is bombarded by energized particles which arrive at a normal incidence to the substrate and produce an anisotropic etch.
While plasma etching offers a valuable tool in performing precise etching of various substrates at the decreased dimensions required by current circuit integration standards, plasma etching suffers from a variety of problems. One problem in particular is that of maintaining the glow discharge during plasma ignition. Often, it is necessary to employ high pressures and/or greatly increased gas flow rates in order to maintain the glow discharge plasma within the etching chamber. Unfortunately, an associated drawback of using higher pressures and increased flow rates is damage to the patterned profiles. Inability to maintain the glow discharge, especially in multi-step etching processes, due to reductions in pressure or decreased flow rates leads to significantly decreased process efficiency and increased unit costs.
Thus, there is a need in the art for plasma etching methods which are able to maintain a glow discharge while also operating at lower pressures and without increased gas flow rates.